1. Field of the Invention
The present invention concerns a prosthetic disc that can be identical to the capsule claimed in the above-cited application, except for modifications in the fluid-filled bladder and its fluid. Like the discs of that application, a pair of the novel discs can be implanted to repair a degenerated disc of the spine of a vertebrate, especially the spine of a human being.
2. Description of Related Art
The normal intervertebral disc has an outer ligamentous ring called the anulus which inds the adjacent vertebrae together and is constituted of collagen fibers that are attached to the vertebrae and cross each other so that half of the individual fibers will tighten as the vertebrae are rotated in either direction, thus resisting twisting or torsional motion. Torsional movement between vertebral segments is further restricted by the facet joints.
Deep inside the anulus lies a nucleus pulposis of loose tissue which is slippery and slimy (having about 85% water content) and moves about during bending from front to back or from side to side. Thus, as the opposing surfaces of the vertebrae alter their parallel relationship to each other with bending, the nuclear tissue moves about to fill up the change in distance (wedging) that occurs in the opposing ends of the disc space. With bending, the anulus will bulge on the downward wedged side and be stretched tightly on the upward wedged side.
A classical disc herniation occurs when the anular fibers are weakened or torn and the inner tissue of the nucleus becomes permanently bulged, distended, or extruded out of its normal anular confines. Leg pain in such cases results from this nuclear tissue (or an intact, weakened, bulging anulus) compressing a nerve which passes outward from the spinal canal to the leg.
A major cause of persistent, disabling back pain takes place when the anulus becomes chronically inflamed by a degenerative process. Small nerves that come from branches encircling the outside of the anulus penetrate for a short distance (perhaps 6 to 8 mm) into the anular fibers. Constant abnormal motion between the fiber layers of the anulus, due to loss of bonding between them, may stretch and grind the tiny pain fiber nerve endings. Thus the patient becomes sensitive to the slightest movement. These cases require some form of mechanical limitation to intervertebral disc motion at the painful segment. For the most persistent cases, bony fusions are often performed to stop the painful motion by permanently locking the vertebrae together. In many cases it may be preferred to allow some minor movement (less than that which causes the pain). Preserving some movement helps to prevent mechanical breakdown at nearby segments. At present, to attempt to make and maintain these flexible fusions is not reliably feasible.
Whenever the nuclear tissue is herniated or removed by surgery, the disc space will narrow and lose much of its mobility. Considering that rotation is potentially destructive to the anulus and nucleus, rotation should be limited by any prosthetic device to replace the removed, herniated or degenerated disc, preferably while allowing bending, especially forward and backward. Lateral bending is of lesser importance.
Although we are not aware of any means currently being used to preserve both the height of the disc space and important motions of the vertebral segment, a number of patents describe prosthetic discs that are said to be useful for those purposes. For example, U.S. Pat. No. 3,875,595 (Froning) shows a prosthesis shaped to replace the entire nucleus pulposis of an intervertebral disc. Froning's "prosthesis is a hollow, flexible bladder-like member which is filled with a fluid and/or plastic under adjustable pressure. The pressure may be increased or decreased while the prosthesis is in place over a period of time to determine by trial and error the optimum pressure, and thereupon the stem of the prosthesis is removed. The optimum pressure is maintained over an indefinite period of time by providing an inflating fluid or plastic having properties for holding fluid or water under pressure normally occurring within the disc sufficient to avoid depletion of the inflating contents, a feature which would duplicate the feature of the normal disc" (col. 1, lines 30-43). Froning's prosthesis has stud-like protrusions which fit into sockets that have been forced through the bony end plates of the adjacent vertebrae to anchor the prosthesis against slippage.
U.S. Pat. No. 4,349,921 (Kuntz) shows an intervertebral disc prosthesis formed from any biologically acceptable material such as high density polyethylene, polymethacrylate, stainless steel, or chrome cobalt alloy and dimensionally shaped to replace a natural disc. One of the longitudinal ends of the prosthesis can have a raised flange to facilitate handling and to prevent penetration to an excessive depth, while the other longitudinal end is wedge-shaped to facilitate insertion. The superior and inferior faces are provided with surface characteristics such as grooves, corrugations, or projections to produce a "friction-fit" and are convex to correspond to the adjacent vertebral surfaces.
U.S. Pat. No. 3,867,728 (Stubstad et al.) shows intervertebral disc prostheses of a variety of constructions. Each of these prostheses has a core made of elastic polymer, e.g., a reinforced resilient block of elastomer such as silicone rubber or polyurethane, and a covering providing an outer surface of an open-pore tissue-ingrowth-receptive material. While most of the illustrated prostheses are single elements of a shape approximating that of a human disc, "another version includes a plurality of flexible, curved, bar-like elements with configurations which allow them to lie side by side so as to occupy the interior space of a natural disc from which the nucleus pulposus has been removed" (penultimate sentence of Abstract). See FIGS. 23 and 24. To repair a ruptured disc with either a single or a multi-element prosthesis, the interlaminar space is posterIorly exposed and "laminectomy is performed to gain better access to the disc space and to provide an opening . . . through which the nucleus pulposus will be removed and the prosthesis inserted. The spinal dura and nerve root are identified, the root is dissected free, and together these are retracted laterally to expose the herniation" (col. 14, lines 17-27). After cleaning out the ruptured disc to create a space to receive the prosthesis, "the end plate that will be adjacent to the ingrowth surface of the prosthesis is scraped clean of loose tissue and left with a bleeding surface to promote fixation of the prosthesis by tissue ingrowth" (col. 14, lines 35-39). If the two-element prosthesis is used, "The two segments may be further stabilized by tying them together by cords 129 or by suturing to one of the adjacent vertebrae or other available tissue" (col. 14, lines 48-50).
Intervertebral disc prostheses which are mechanically fastened between vertebrae are shown in U.S. Pat. No. 4,554,914 (Kapp et al.); No. 4,309,777 (Patil); No. 3,426,364 (Lumb); and No. 4,636,217 (Ogilvie et al.).
Vascular circulation and nerve supply to the disc is limited to the outer layers of the anulus, never penetrating more than several millimeters. Most of the nutrition to the inner anulus and nucleus is provided by diffusion throughthe end plates of the vertebral bodies, these bones being quite vascular. Thus, the central disc is the largest avascular and non-innervated structure of the body. A variety of degenerative changes may occur if the vertebral end plates become sclerotic (hardened). Nutrition to the inner disc slowly ceases, resulting in nuclear and anular fiber degeneration, shrinkage of the nucleus, segmental laxity, spur formation, disc space collapse, spontaneous fusion, and other changes.
The physiological isolation of the central disc is normally so complete, relative to circulating body fluids and chemicals, that throughout lifetime, provided the disc remains intact, protein fractions, enzymes and other compounds peculiar only to the inner disc are never exposed to the rest of the body constituents. As a result, when the disc anulus is torn, hidden internal discal constituents may suddenly become exposed to the general circulation. In many cases, antigen-antibody reactions may thus begin, as the circulating immune system is exposed to new, "foreign" tissues. Frank allergic reactions to autologous disc tissue have been documented in controlled animal experiments.
A further situation of greater importance may also occur, namely, a combined, chronic mechanical and chemical irritation of the fine free nerve endings found in the outer disc layers. Anular cracks permit nuclear materials to reach these nerve endings; shearing forces between delaminating layers of anular fibers irritate the traversing, penetrating free nerve endings. These irritations lead to mechanically induced acute, and postural, chronic pain perception arising from the outer rind of the disc. Since the anterior one-third of the disc in inervated by deep sympathetic fibers and the posterior two-thirds by somatic sensory fibers, the combined circular irritation produces a highly disagreeable and often disabling pain. At the present time, the primary treatment for such common pain is (a) alteration of life style, (b) reduction in overall activity level, (c) extensive exercise program, (d) use of antiinflammatory medication, (e) surgical or enzymatic discectomy, or (f) stoppage of mechanical motion by bony fusion.